Aqueous solutions containing ammonium compounds and especially "fixed ammonia" and optionally acid gases are a common wastewater which requires purification. Such a wastewater is produced as a by-product from coal carbonization plants. The need for such purification has become increasingly important due to the emphasis today on clean air and clean water. If the wastewater is to be discharged into a river or a stream, it is obviously necessary to reduce the noxious or toxic properties of the wastewater. On the other hand, in a coal carbonization plant it is often desirable to reuse the wastewater to "quench" the hot coke from the coke ovens. If these wastewaters contain noxious or toxic materials, then there may be a serious air pollution problem.
A common way of removing impurities from these wastewaters is by means of a two-step distillation process using distillation apparatus with a free ammonia and a fixed ammonia section. See "Industrial Chemistry", E. R. Riegel, Reinhold Publishing Corp., N.Y., 1942, pp. 265-268.
One serious problem of this two-step distillation process is the difficulty encountered in removing acid gases down to very low levels in the first distillation step, and this problem results because vapors from the second distillation, used as stripping steam in the first distillation, contain ammonia which reduces the stripping ability of the steam. One solution to this problem, described in U.S. Pat. No. 4,260,462, incorporated herein by reference, is to exclude the ammonia removed in the second distillation from entering the first, thereby maintaining a low pH in the first distillation which enhances the ability to remove acid gases in this first distillation. U.S. Pat. No. 4,260,462 describes a method using indirect heat exchange whereby the energy in the (ammonia-containing) steam from the second distillation can be recovered to generate steam for use in the first distillation. By allowing reuse in the first distillation of steam from the second distillation and the addition of various other energy saving techniques, the ammonia distillation process in U.S. Pat. No. 4,260,462 provides substantially improved ammonia removal when compared to the conventional process, and at substantially reduced energy usage.
The term "fixed ammonia salts" is used herein in its generally accepted meaning to cover those compounds of ammonia which are not decomposed by heat alone at the temperature employed in the ammonia still, but which in alkaline solution yield free ammonia. Generally, in coke-plant wastewaters, the major portion of the fixed ammonia is present as ammonium chloride. Other such compounds are ammonium thiosulfate, ammonium thiocyanate, ammonium ferrocyanide and ammonium sulfate. The term "fixed ammonia" refers to the ammonia portion of the fixed ammonia salts.
The term "free ammonia salts" as used herein is meant to include ammonia itself and also those compounds present in wastewaters from which ammonia is liberated by heat in the free ammonia still. Such compounds are ammonium carbonate, ammonium bicarbonate, ammonium sulfide, ammonium bisulfide, ammonium cyanide and ammonium carbamate. The term "free ammonia" refers to ammonia or the ammonia portion of the free ammonia salts.
The term "lime" is used herein to include any of the various chemical and physical forms of quicklime (calcium oxide), hydrated lime (calcium hydroxide), and hydraulic lime. The lime may include a number of impurities such as silica, calcium carbonate, magnesium oxide, iron oxide, and aluminum oxide. Some of them may be insoluble when added to the wastewater.
The term "acid gases" is used herein to include hydrogen cyanide, hydrogen sulfide and carbon dioxide. These gases may be present in the original wastewater as weak ammonium salts that completely dissociate into ammonia and their respective acid gases upon steam distillation.
The wastewaters described herein are generally industrial wastewaters produced by the high-temperature cracking of carbonaceous material. Often such carbonaceous materials are coal or petroleum products. Wastewater may also be formed in water-scrubbing of coke-oven gases.